The invention relates to a method for treating black liquor at a pulp mill in order to recover chemicals and energy contained therein. Further, the invention relates to equipment for treating black liquor at a pulp mill in order to recover chemicals and energy contained therein.
In the pulping process, wood raw material, such as chips, is treated by cooking it in a chemical solution containing lye, among other things. After cooking, fibre pulp detached from the wood material is separated from the cooking liquor, in which various components of wood material, such as lignin, dissolved during cooking will remain. The chemical mixture separated after cooking, i.e. black liquor, is evaporated in an evaporation plant in order to obtain combustible material containing as little water as possible. The dry solids content of the material from the last phase of the evaporation plant, and conventionally introduced into a soda recovery boiler for burning, may be up to 85%.
Conventionally, black liquor is burnt in a soda recovery boiler, which results in obtaining heat, whereby water is heated and vaporized for producing energy, and the salt melts and cooking chemicals are reproduced therefrom. This solution is presented, for instance, in Finnish Patents 82494 and 91290. Attempts have been made to replace the soda recovery boiler, for instance, by gasifying black liquor, but in practice, a commercially competitive solution has not yet been achieved.
WO publication 2004/005610 sets forth a solution, in which black liquor is pyrolysed and the coke produced in pyrolysis is gasified. However, this process is difficult in practice and it requires separate, expensive gasification equipment.